Opto-electronic integrated circuits (OEICs) will increasingly be used in telecommunications, computing, and signal processing. Several applications for OEICs require the optical separation of the two orthogonal polarization modes carried on a single-mode waveguide in the OEIC. An example of such an OEIC is a polarization-diversity receiver which splits the incoming light into two beams propagating on separate waveguides according to the polarization of the light. It then separately detects the intensities of both polarization modes. The two modes are represented alternatively as the TE and TM modes or as the H and V modes. In the TE or H mode, the electric vector of the propagating light is parallel to the top surface of the waveguide while in the TM or V mode it is perpendicular. A generic 2.times.2 polarization beam splitter 10 is illustrated schematically in FIG. 1. A first input waveguide 12 carries orthogonal signals H and V while a second input waveguide 14 carries orthogonal signals H' and V'. The polarization beam splitter 10 preferentially acts on the orthogonal polarizations such that a first output waveguide 16 carries the signals H' and V and a second output waveguide 18 carries the signals H and V'. If the second input waveguide 14 is disregarded, then the H and V input signals on the first input waveguide 12 are split according to polarization onto the two output waveguides 18 and 16 respectively. This would be a 1.times.2 splitter.
The devices described herein are reciprocal devices so that a splitter operated in the reverse direction is a combiner. For example, the above 1.times.2 splitter is also a 2.times.1 polarization combiner receiving H and V input signals from the right on waveguides 18 and 16 respectively and outputting both H and V signals on the first waveguide 12. Hereinafter, a splitter will be understood to encompass a combiner.
A common integrated type of 2.times.2 polarizing beam splitter is a directional coupler as, disclosed, for example, by Cheung et al. in U.S. Pat. No. 5,002,349 for LiNbO.sub.3 substrates. Smith et al. reported some of the same work in "Polarization-independent acoustically tunable optical filter," Applied Physics Letters, volume 56, 1990, pp. 209-211. This splitter has two continuous optical waveguides which are brought closely together in an interaction region in which the TM mode but not the TE mode spatially oscillate from one waveguide to the other. Thus, if the length of the coupling region is made to be exactly one coupling length, the TM modes are exchanged between the two waveguides. However, the degree of coupling depends strongly on the small separation between the waveguides, and, hence, it is difficult to make the coupling region of exactly the right length. A directional coupler can be tuned electrostatically, but a completely passive and accurate polarizing beam splitter is desired.
Goto et al. have disclosed an adiabatic Y-branch polarizing splitter in "A TE-TM mode splitter in LiNbO.sub.3 by proton exchange and Ti diffusion," Journal of Lightwave Technology, volume 7, 1989, pp. 1567-1574. This device amounts to a 1.times.2 polarization beam splitter. In it, the end of a first waveguide is obliquely butted up to the side of second waveguide carrying the two polarization states (modes). The butted structure is adiabatically configured such that the fundamental mode of each polarization state adiabatically evolves into that waveguide which has the highest effective index for that polarization state. It is, however, not clear how to apply the device of Goto et al. to a 2.times.2 splitter. Shani et al. have disclosed a similar adiabatic 1.times.2 splitter in "Integrated optic adiabatic polarization splitter on silicon," Applied Physics Letters, volume 56, 1990, pp. 120-121.
Izutsu et al. have disclosed another type of beam splitter in "Optical-waveguide hybrid coupler", Optics Letters, volume 7, 1982, pp. 549-551. In this device, two waveguides cross at an adiabatic X-junction, and their widths asymmetrically change at the junction. Neyer discloses another, non-adiabatic X-junction beam splitter in "Low-crosstalk passive polarization splitters using Ti:LiNbO.sub.3 waveguide crossings," Applied Physics Letters, volume 55, 1989, pp. 927-929. In this device, the polarization splitting function is achieved similarly to the directional coupler.